cgroup-util: drop 'controller' argument from cg_enumerate_subgroups()

Nowadays it always takes SYSTEMD_CGROUP_CONTROLLER as controller.
Let's drop it.

No functional change, just refactoring.

src/oom/oomd-manager.c

@@ -210,7 +210,7 @@ static int recursively_get_cgroup_context(Hashmap *new_h, const char *path) {
         assert(new_h);
         assert(path);
 
-        r = cg_enumerate_subgroups(SYSTEMD_CGROUP_CONTROLLER, path, &d);
+        r = cg_enumerate_subgroups(path, &d);
         if (r < 0)
                 return r;